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6-mercaptopurine (6-MP) is metabolized to the inactive nucleotide metabolite 6-methylmercaptopurine(6-MMP), and to the active metabolite6-thioguanine (6-TG), which has cytotoxic and immunosuppressive activity. 6-MMP is produced by the enzyme thiopurine methyltransferase (TPMT); persons homozygous (0.3% of the population) or heterozygous (11% of the population) for a mutant allele of TPMT that results in an inactive enzyme areat risk for leukopenia. To determine whether therapeutic 6-MP metabolite levels correlate with medication-induced toxicity and TPMT genotype, researchers studied therapeutic responses in 92 pediatric patients (younger than 19) with inflammatory bowel disease (IBD) who had received 6-MP or its prodrug azathioprine for at…